This invention relates to electrostatic spray devices having one or more high voltage electrodes for charging coating material to be sprayed upon an object, and, more particularly, to an electrostatic spray gun adapted to receive a cable assembly which adjustably mounts a combined electrode and particle deflector in the path of coating particles discharged from the spray gun to control the spray pattern and electrostatic charge applied to the coating particles while minimizing shock and ignition hazard due to inadvertent discharge of electrical energy capacitively stored in the gun.
In industrial finishing applications, coating particles are emitted from a spray device such as a spray gun toward an object to be coated. One type of coating material is in the form of particulate powder entrained in a stream of air. One or more electrodes associated with the spray gun impart an electrostatic charge to the coating particles as they are emitted toward an object to be coated. The object to be coated is maintained at an electrostatic potential different than that of the charged coating particles so that the coating particles are attracted to the article and deposited thereon with improved efficiency and coverage.
The spray gun is connected to a high voltage electrostatic supply which provides electrostatic potentials of approximately 30 KV or more to the charging electrode. The high voltage electrostatic supply may be remotely located with respect to the spray gun, in which event an electrical cable insulated for high voltage is connected between the spray gun and the remote source. A device of this type for electrostatically spraying particulate powder material is shown, for example, in U.S. Pat. No. 3,746,254 to Duncan et al.
In electrostatic spray coating systems, electrical energy is capacitively stored in the electrical path which supplied charging potential to the electrode. Included in this charge conducting path are components of the high voltage electrostatic supply, interconnecting high voltage cables, electrical switches, contacts, conductors and the like. Additionally, electrical energy is capacitively stored in the spray gun itself as a consequence of the presence of structural elements of an electrically conductive nature which function in much the same manner as plates of a capacitor. Should the capacitively stored energy be rapidly discharged, such as where the elecrode is inadvertently electrically grounded or brought in close proximity to an electrically grounded object, a spark can result having sufficient energy to cause ignition of combustible concentrations of coating powder which are often present in the environment surrounding the spray gun. Additonally, inadvertent discharge of electrically stored energy can create shock hazards to personnel who come in contact with the charging electrode, particularly when using hand-held spray guns.
To reduce the rate of discharge of capacitively stored energy in the foregoing situations to safe limits, it has been the practice in the prior art to connect one or more discrete resistors in the high voltage path which interconnects the charging electrode and the high voltage electrostatic supply. Usually, at least one relatively large resistor, and in some cases a second resistor of lesser value, are incorporated in the high voltage path within the spray gun upstream of the electrode. See, for example, U.S. Pat. Nos. 4,182,490 to Kennon; 4,335,851 to Hastings; and 3,599,038 to Skidmore.
One problem in the utilization of discrete resistors contained in prior art spray guns upstream of the electrode is that a mechanical joint or connection must be established between the high voltage cable and the resistor(s) within one or more passageways formed in the spray gun. Such connections are often metallic and are characterized by sharp edges giving rise to a corona discharge which can be transmitted through the gun wall to a grounded object. If the gun wall is insulated, repeated corona discharge causes dielectric breakdown of the insulation and the subsequent danger of a spark being transmitted through the gun wall to a grounded object.
The danger of corona discharge in prior art spray guns of this type has been reduced by injecting dielecric grease into the passageway(s) in the spray gun in an effort to encase the conductive metallic connection between the cable and resistor(s) with such dielectric grease. See U.S. Pat. Nos. 4,534,106 to Simashkevish et al and 4,543,710 to Hastings et al. While this reduces the risk of conrona discharge, it is often difficult for the user of the gun to ensure sufficient dielectric grease has been deposited into the passageway carrying the cable and resistor after these elements have been removed or replaced for routine maintenance. Many users do not have the facilities, or fail to take the time, to test the insulative properties of the spray gun after a resistor has been replaced. However, any air gaps in the area of the cable and resistor connection which are not filled with dielectric grease can cause corona discharge and this increases the hazard of ignition in the explosive environment surrounding the spray gun and/or a danger of shock to the operator.
Electrostatic spray guns, particularly those designed for spraying particulate powder material, also include a particle deflector mounted at the nozzle end of the spray gun. In one preferred form, the parricle deflector is in the shape of a cone which is mounted axially in the flow path of the particulate powder material sprayed from a discharge opening in the nozzle of the spray gun. The particles impact the cone and are deflected radially outwardly into a conical spray pattern for deposition onto an article to be coated. Structure may or may not be provided to adjust the axial position of the conical particle deflector with respect to the discharge opening of the nozzle of the gun. If adjustment is permitted, the width of the spray pattern can be varied for a given application. Examples of electrostatic powder spray guns employing adjustable deflectors are shown, for example, in U.S Pat. Nos. 3,521,815 to Szasz and 3,608,823 to Buschor.
It has been disclosed in co-pending U.S. patent applications, Ser. No. 724,392 entitled "Powder Spray Gun", filed Apr. 18, 1985 by Sharpless et al, and owned by the same assignee as this invention, and a continuation-in-part of that application, Ser. No. 791,352 filed Oct. 25, 1985 and entitled "Particle Spray Gun", that in order to maximize the "transfer efficiency", or the efficiency with which charged coating particles sprayed from the gun are deposited on an article to be coated, it is advantageous to form the particle deflector with a resistive sheet between its forward and rearward ends leaving exposed only the perimeter of the sheet at the periphery of the particle deflector. The resistive sheet is electrically connected to a high voltage electrostatic cable and its perimeter functions as a multi-point electrode which is positioned by the particle deflector proximate the discharge opening of the nozzle of the gun directly in the path of the particle stream. Other examples of electrostatic spray guns employing an electrode connected to or carried by a particle deflector include U.S. Pat. No. 3,521,815 to Szasz and U.K. Patent No. 1,406,358.
Although it is believed that the transfer efficiency is increased by the designs disclosed in the '392 and '352 applications, and perhaps by those disclosed in the '815 Szasz patent and '358 U.K. patent, ignition and shock hazards remain to some extent with each of these designs. The systems disclosed in the pending applications, and in the U.K. patent, each employ discrete resistors mechanically connected to an electrical cable upstream from the charging electrode within a passageway formed in the spray gun, which, for the reasons given above, can give rise to corona discharge resulting in an explosion or shock hazard. The spray gun disclosed in the Szasz patent apparently employs no resistors either in the gun or the cable which form the high voltage path between the charging electrode and electrostatic supply. This appears to present a serious problem of inadvertent rapid discharge of energy capacitively stored either in the components of the high voltage electrostatic supply, high voltage cable or the structure of the spray gun itself.